Highly radioactive water has been found at a second reactor at a crippled nuclear power station in Japan, the plant's operator said, as fears of contamination escalated two weeks after a huge earthquake and tsunami battered the complex.

Underscoring growing international concern about nuclear power raised by the accident in northeast Japan, U.N. Secretary-General Ban Ki-moon said in a statement it was time to reassess the international nuclear safety regime.

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Earlier, Japanese Prime Minister Naoto Kan, making his first public statement on the crisis in a week, said the situation at the Fukushima nuclear complex, 240 km (150 miles) north of Tokyo, was "nowhere near" being resolved.

"We are making efforts to prevent it from getting worse, but I feel we cannot become complacent," Kan told reporters. "We must continue to be on our guard."

The comments reflected a spike of unease in Japan after several days of slow but steady progress in containing the nuclear accident, which was triggered by a devastating earthquake and tsunami on March 11.

The 9.0 magnitude quake and giant waves it triggered left more than 10,000 people dead and 17,500 missing.

Despite such a shocking toll, much attention since the disaster has been on the possibility of a catastrophic meltdown at Fukushima.

Two of the plant's six reactors are now seen as safe but the other four are volatile, occasionally emitting steam and smoke.

More than 700 engineers have been working in shifts to stabilize the plant and work has been advancing to restart water pumps to cool their fuel rods.

But fresh fears were raised on Thursday when three workers trying to cool the most critical reactor were exposed to radiation levels 10,000 times higher than normally found in a reactor. Two of them suffered radiation burns when contaminated water seeped over their shoes.

The high level of contamination raised the possibility of a leak of radioactive material through a crack in the core's container which would mean a serious reversal following slow progress in getting the plant under control.